In general everyday use, railcars collide together frequently. For example, the railcars of a train in motion generally bump into each other when the train slows or stops. Also, railcar collisions occur when assembling railcars into a train. The difference in velocity between the railcars in such collisions is typically low. However, due to the large mass of the railcars, the railcars collide with sufficient impact energy, unless otherwise absorbed, to cause damage to the railcars and any cargo carried by the railcars even in these collisions at low velocity differences. To absorb the impact of normal railcar collisions, a railcar generally includes cushion devices mounted at each end of the railcar between the railcar and its couplers. (In some railcars, a centrally mounted cushion device and sliding sill are used.)
Currently in common use on railcars are hydraulic cushion units which generally comprise a piston within a cylinder barrel filled with a hydraulic fluid, typically oil. In general, the devices can be described as non-linear hydraulic shock absorbers. In a railcar collision, the piston is displaced through the barrel. As the piston travels through the barrel, the hydraulic fluid in the barrel is compressed by the piston, forcing the fluid through orifices in the cylindrical wall of the barrel. The action of forcing the fluid through orifices acts to absorb impact energy by heating the fluid. Generally, the amount of force that can be translated into heat energy is proportional to the square of the piston velocity.
Typically, hydraulic cushion units are configured to absorb a constant force throughout the piston stroke by varying the number of orifices through which the fluid vents as the piston is displaced. More specifically, the orifices are distributed along the length of the barrel. Therefore, during the course of the piston's travel through the barrel, the piston bypasses orifices one (or more) by one, leaving fewer and fewer orifices through which the fluid can vent. When the force absorbed by a cushion device is maintained substantially constant, the rate of change of acceleration is minimal. Thus, this configuration serves to minimize sudden changes in velocity or "jerking" motions of railcars connected in a train. After absorbing an impact, the piston is returned to its initial position in the barrel of the cushion device by mechanical springs or a gas charged device.
In the typical operating environment, railcar cushion devices are subject to failure, particularly in the hydraulic seals, from the wearing of moving parts and from rust and corrosion. Failure can also result from the stress of impacts greater than the rated capacity of the devices. To assure proper functionality of the devices, the performance of the devices is periodically tested.
Various test methods are known. Some test methods involve visual inspection for noticeable signs of impairment, e.g. rust, breaks, and leaked hydraulic fluid. However, failures frequently occur without producing noticeable signs of impairment. Therefore, with visual inspection alone, many failures remain undetected. Also, the cushion devices may be difficult to inspect visually while installed on a railcar.
Another type of test, referred to herein as range of motion testing, involves moving the cushion devices through a normal range of motion to detect failures that result in binding or obstruction of free movement. This type of test can detect impairments that are not visually apparent, but fails to detect abnormal cushion device operation short of binding. It is possible to perform this type of test on an installed cushion device.
Most other test methods are of a type, referred to herein as impact testing, that involves exposing a cushion device to kinetic energy and then analyzing the forces produced by the cushion device to detect nonconforming operation. A variety of means for exposing the cushion device to kinetic energy are known.
One such means is a drop hammer. In general, a drop hammer comprises a hammer member and an anvil member. The cushion device is removed from the railcar and mounted on top of the anvil member. The hammer member is raised on a vertical frame or track above the cushion device and anvil member by pulleys or the like. The hammer member is then allowed to drop on the cushion device from a predetermined height above the cushion device thereby applying a known amount of kinetic energy to the cushion device. The forces produced by the cushion device are measured using a load cell comprising a pressure transducer, a piston, and a fluid filled cylinder in the anvil member.
One disadvantage of the drop hammer is that it requires removal of the cushion device from the railcar. Removing and reinstalling the cushion device is time-consuming and expensive. The railcar must be placed out of service while the cushion device is removed, possibly resulting in lost revenue to the railroad company for the cargo that could have been carried by the railcar during this time. Further, to allow regular testing of cushion devices involving removal of cushion devices from railcars, a railroad company may be required to maintain more rolling stock or a larger stock of replacement cushion devices.
The standard means in the railroad industry of applying kinetic energy to a cushion device involves simply running a rolling railcar into a stationary railcar on which the cushion device is installed. Typically, the rolling railcar is released from a predetermined elevation on an incline so that the amount of kinetic energy is applied to the cushion device can be estimated. The forces produced by the cushion device are typically calculated indirectly from a measurement of the post-collision velocity of the stationary railcar.
The disadvantages of this method are that the method is inexact, time-consuming to carry out, not easily repeatable or reproducible, and can result in damage to one or both railcars. More specifically, the amount of kinetic energy applied to the cushion device is generally assumed from the elevation on the incline at which the rolling railcar was released. The actual final velocity of the rolling railcar at the time of the collision, however, is affected by friction and other forces. Typically, there is significant variance in the final velocity between repetitions of the test and for different rolling railcars used in the test. Since kinetic energy is a function of the square of velocity, this variance in final velocity can significantly affect the actual kinetic energy applied by the rolling railcar to the cushion device at the time of the collision. The applied kinetic energy is also a function of the mass of the rolling railcar which typically varies for different rolling railcars used to test cushion devices and may not be accurately known. Results of this test methodology are therefore inexact and not sufficiently repeatable.
In general, prior impact testing methods and apparatus apply a predetermined amount of kinetic energy in an initial impact to a railcar cushion device and analyze the forces produced by the cushion device. However, after the initial impact, the motion of the cushion device is uncontrolled and generally unknown, being dependent on the function or performance of the cushion device itself and other factors. An analysis of the forces produced by cushion devices according to such tests are therefore often not comparable to those of other cushion devices because the applied motion after the initial impact is not identical and not known.
A test method and apparatus is therefore needed to provide repeatable and reproducible testing of railcar cushion devices in a short amount of time with or without removal of the cushion device from the railcar. Further, a test method and apparatus is needed for accurately diagnosing conformance of railcar cushion device performance with rated parameters.